1. Field of the Invention
The present invention relates to a variable frequency oscillating circuit.
2. Description of the Related Art
In semiconductor devices, an oscillating circuit is incorporated in some cases.
In this case, if a plurality of clock signals having different frequencies are required, there may be provided a plurality of oscillating circuits, and a predetermined clock signal is used among the clock signals output from the respective oscillating circuits.
However, provision of the plurality of oscillating circuits leads to an increase in its circuit size.
To deal with this problem, a variable frequency oscillating circuit as shown in FIG. 10 has been proposed. In this circuit, a ring oscillator 12 includes an odd number of inverters 11-2 to 11-n, and transistors 14 and 15 having different threshold voltages and a switch 17 are provided between a power supply line 13 of the ring oscillator 12 and a power supply 21. When a controller 22 is controlled to switch a connection destination of the switch 17, a power supply voltage of the power supply 21 is dropped by the transistor 14 or the transistor 15, and then is supplied to the ring oscillator 12 (for example, see JP 10-190414 A).
With such a configuration, one ring oscillator is used, and the power supply voltage supplied to the ring oscillator 12 is changed to change a frequency of a clock signal output from the ring oscillator 12, with the result that a plurality of clock signals having different frequencies can be generated. Accordingly, a circuit size is not increased.
However, in the technology disclosed in JP 10-190414 A, when the connection destination of the switch 17 is switched, an unintended power supply voltage may be supplied to the ring oscillator 12 to generate a clock signal having an unintended frequency. As a result, a semiconductor device may malfunction.